As a mounting technique of a disk array system, the so-called rack mount type disk array system is known, and in such a rack mount type disk array system, boxes which store the canisters in which a plurality of memory disk units (for example, hard disk drives (HDD)) are assembled into a unit are attached on several stages. The technique concerning the rack mount type disk array system is disclosed in Japanese Patent Laid-Open No. 2004-22058.